1. Field of the Invention
The present invention relates to a communication device, a noise removing method, and a program.
2. Description of the Related Art
In recent years, a GPS (Global Positioning System) function is mounted in various electronic apparatuses, such as a car navigation device, a portable communication device like a mobile phone, or a digital still camera. The electronic apparatus includes a GPS receiving device (an example of a communication device) that receives a transmission signal called a GPS L1 C/A code signal that is transmitted from four or more GPS satellites and specifies a position based on the received transmission signal or functions as a GPS receiving device, thereby realizing the GPS function.
The GPS receiving device that is included in the electronic apparatus demodulates a signal from each GPS satellite to acquire orbital data of each GPS satellite, derives a three-dimensional position of the OPS receiving device by a simultaneous equation using the orbital data, time information, and a delay time of the received signal, and specifies the position. In this case, the GPS receiving device uses orbital data of a plurality of GPS satellites in order to remove an influence due to an error between internal time information of the GPS receiving device and time information in each GPS satellite.
In this case, a transmission signal that is transmitted from each GPS satellite is a signal that is obtained by performing BPSK (Binary Phase Shift Keying) modulation on a carrier of 1575.42 MHz, based on a spectrum spread signal that is obtained by subjecting data of 50 bps to a spectrum spread using a Gold code where a code length is 1023 and a chip rate is at 1.023 MHz. Accordingly, when the GPS receiving device receives a transmission signal from each GPS satellite and demodulates a transmitted spectrum spread signal, it may be requested to synchronize a spread code, a carrier, and data.
A technology that is related to a communication device that receives a transmission signal where a spectrum spread signal subjected to a spectrum spread is modulated has been developed. A technology that quickly performs synchronous capturing of a spread code in a transmission signal transmitted from a GPS satellite is disclosed in Patent Document 1 as one example.
Meanwhile, the electronic apparatus that has a GPS function has had multifunction and high performance, and unnecessary radiation that is generated in the electronic apparatus has increased. The unnecessary radiation that is generated in the electronic apparatus corresponds to an external noise due to a GPS receiving device (or GPS receiving device of an electronic apparatus, hereinafter, this is applicable) that is included in the electronic apparatus. In this case, representative examples that become factors of the external noise due to the unnecessary radiation may include a clock signal that interferes through coupling or space between wiring lines in the electronic apparatus, harmonics due to a high speed signal that passes through a data bus, a variation in load of a circuit, or a variation in power due to a switching regulator.
When the external noise is applied to an analog circuit of the GPS receiving device, if the external noise has the same level as a normal thermal noise (for example, −111 [dBm] when the thermal noise is converted into a bandwidth of 2 MHz) that is generated in the GPS receiving device, the OPS receiving device can normally demodulate the spectrum spread signal. However, when the external noise is stronger than the normal thermal noise that is generated in the GPS receiving device, reception sensitivity is deteriorated by the amount exceeding the level of the normal thermal noise. If the reciprocal of a “ratio between a received transmission signal and a thermal noise+an external noise” (S/(N+I)) approximates to a process gain, the GPS receiving device may not normally demodulate the spectrum spread signal.
Accordingly, for example, as in the GPS receiving device, in a communication device that receives a transmission signal where a spectrum spread signal subjected to a spectrum spread is modulated, an influence due to the external noise needs to be removed as much as possible, in order to normally demodulate the spectrum spread signal.
Among them, a technology for removing an external noise in a communication device that receives a transmission signal where a spectrum spread signal subjected to a spectrum spread is modulated has been developed. In order to remove the external noise in the GPS receiving device, a technology using a notch filter (it may also be referred to as a band-elimination filter) is disclosed in a Non-Patent Document 1 as one example,    [Patent Document 1] JP-A-2003-232844    [Non-Patent Document 1] Daniele Borio, Laura Camoriano, Paolo Mulassano, “Analysis of the One-Pole Notch Filter for Interference Mitigation: Wiener Solution and Loss Estimation”, ION GNSS 19th International Technical Meeting of the Satellite Division, 26-29 Sep. 2006, pp. 1849-1860.
The communication device where the technology for removing an external noise according to the related art is applied includes a notch filter that attenuates a signal corresponding to a set notch frequency. The communication device controls the notch frequency of the notch filter, thereby removing the external noise that is generated due to the unnecessary radiation. In this case, the communication device where the technology for removing an external noise according to the related art is applied uses an LMS (Least Mean Square) algorithm that is generally used in an adaptive filter, thereby controlling the notch frequency of the notch filter. However, in the LMS algorithm where the technology for removing an external noise according to the related art is used, in order to control the notch frequency, a feedback operation needs to be performed on a signal that is used to remove the external noise. For this reason, in the communication device where the technology for removing an external noise according to the related art is applied, it is necessary to consider convergence as a control loop. That is, in the communication device where the technology for removing an external noise according to the related art is applied, an unstable operation, such as divergence, may be generated depending on setting of the control loop or a signal (signal used to remove the external noise) input to the notch filter.
Accordingly, the communication device using the technology for removing an external noise according to the related art cannot stably remove the external noise. Further, no description is given to a method of removing the external noise in the communication device according to the related art that quickly performs synchronous capturing of a spread code. Even though the technologies according to the related art are combined, it may not be possible to stably remove the external noise.
Further, as a countermeasure that removes an influence due to the external noise as much as possible, for example, a shield material or a shield case is used in the communication device (or electronic apparatus where the communication device is mounted), thereby suppressing unnecessary radiation. However, since the above countermeasure affects a design or cost of the communication device (or electronic apparatus where the communication device is mounted), the above countermeasure is not preferable.